1. Field of the Invention
The present invention relates generally to improvements in the art of ballasting vessels and, more particularly, but not by way of limitation, to the ballasting of drilling platforms.
2. Brief Discussion of the Prior Art
When a vessel; for example, a ship or a drilling platform, is at rest in a body of water, it is in a condition of mechanical equilibrium under the influence of several forces that are exerted upon it. These forces include the weight of the vessel including loads the vessel may be carrying, the buoyant force on the vessel and, in the case of a submersible drilling platform, constraint forces that the seabed exerts upwardly on pads at the lower ends of the platform legs when the vessel is submerged to the seabottom.
For the equilibrium to exist, Newton's laws of motion require that two conditions be met: (1) there must be no net force on the vessel and (2) there must be no net torque on the vessel about any axis drawn thereon. Since these conditions are the direct consequence of natural laws, it follows that the vessel will position itself in the water or on the seabed so that the two conditions are met. For example, if the distribution of the weight and buoyant force on a ship are such that the two conditions can be met only if the ship floats on its side, the ship will capsize if placed in the water. Similarly, a drilling platform will rise off the seabed or dig on a leg into the seabed if such is required to cause the two conditions to be met.
Ballasting is used to achieve a concurrence of the two conditions of equilibrium with other conditions on the state of the vessel that are imposed by the vessel user. Perhaps the most widely imposed condition is that on the position of a ship in the water; ballasting is used to cause the conditions of equilibrium to be met when the ship is riding on even keel with a selected draft. However, ballasting can also be used to impose other conditions on other types of vessels. For example, in the drilling of an offshore well from a submersible drilling platform, ballasting is often used to achieve an even distribution of the forces between the pads on the legs of the drilling platform and the seabed.
In general, the two conditions of equilibrium and the additional, imposed, conditions, whatever these conditions might be, can be concurrently satisfied by appropriately positioning the combined center of gravity of the vessel with its contents, including ballast, and by attaining an appropriate total weight for the vessel with its contents and ballast. Thus, in the first of the two examples above, the ship would be ballasted so that the combined center of gravity of the ship and ballast would be disposed directly above the ship's keel and the total weight for the ship and ballast would be selected to equal the buoyant force on the ship at the selected draft. In the case of the drilling platform, the location of the center of gravity of the ballasted platform and the weight of the platform and ballast would be selected in accordance with circumstances at hand. For example, during drilling operations, these quantities might be periodically adjusted to limit shifts in the location of the center of gravity of the platform, including drill pipe it carries, as the pipe is moved from storage and added to the drill string. On the other hand, if the platform is to be moved to a new location, an appropriate initial choice for these quantities, preparatory to lifting the platform from the seabed, would be a generally central location of the center of gravity on the platform and a total weight equal to the buoyant force on the platform. With this choice, a further reduction of the total weight of the platform and its ballast can be used to lift the platform off the seabed without causing excessive tilting of the platform. In any event, the appropriate location for the vessel and ballast center of gravity and the total vessel and ballast weight can be determined by standard techniques prior to a commencement of any adjustment to the ballast configuration.
In the past, the ballasting of a vessel has been often a matter of trial and error. For example, where the vessel is afloat so that the effect of a ballast change can be immediately assessed by the person carrying out the ballasting operation, the usual approach is to add or remove ballast from various ones of a set of ballast tanks with which the vessel is provided while observing the effect of each ballast change until the vessel is in trim with a selected draft. A problem with the trial and error approach in these circumstances is that a large number of ballast tanks on the vessel might be partially filled when the ballasting operation is completed. Thus, when the vessel rolls, the ballast in those tanks will shift and thereby reposition the combined center of gravity of the vessel and ballast. This repositioning of the total center of gravity destabilizes the vessal and, where enough tanks are partially filled, will bring the vessel to an unstable condition.
Similarly, difficulties arise with the trial and error approach where a vessel, such as an offshore drilling platform, is partially supported by the seabed. In this case, the effect of a ballast adjustment will not be visible so that a different trial and error approach is used. Generally, the ballaster will determine the total weight the vessel and ballast must have and the location of the combined center of gravity of the vessel and ballast to meet his ends and then find a particular ballast configuration that will result in the so-determined weight and center of gravity of the vessel and ballast. In finding this configuration, he will use his intuition to select an initial trial ballast configuration from which he can calculate a center of gravity and weight for the ballasted vessel. The calculated center of gravity and weight are then compared to the center of gravity and weight the ballasted vessel is to have and the trial ballast configuration adjusted accordingly. This procedure continues until the ballaster arrives at a ballast configuration for which the total weight of the vessel and ballast and the center of gravity of the vessel and ballast match the weight and center of gravity that the ballasted vessel is to have within acceptable tolerances.
One problem with this approach is that the time and effort the approach involves depends upon the intuition of the ballaster. A number of trial ballast configurations, each requiring a center of gravity and weight determination, may be required to find a ballast configuration that will result in the necessary match. In most cases, the final determination of a proper ballast configuration is the result of a series of calculations the complexity of which presents opportunity for human error. Such errors can be disastrous. For example, if an offshore drilling platform is being ballasted to bring the platform to neutral buoyancy preparatory to lifting the platform from the seabed, the platform can suddenly break free from the seabed when an attempt is made to achieve a free floating condition with the result that the platform can capsize or rise on a tilt that will cause large scale shifting of equipment on the platform. In this regard, it should be noted that the trial and error approach to arriving at a proper ballast configuration for a vessel partially supported by the seabottom provides no information to the ballaster in regard to the manner in which the ballast configuration is to be achieved. That is, once the appropriate ballast configuration has been determined, the ballaster must again rely upon his intuition to bring the vessel to this configuration. In some circumstances, an intermediate ballast configuration may be unsafe even though the final ballast configuration determined by the trial and error approach may be safe. Thus, any lapse in the judgment or calculations of the ballaster can have consequences that bear on the safety of the crew of the vessel and on the structural integrity of the vessel and its load.